1. Field of the Invention
The invention concerns a method and a corresponding apparatus for continuously desalinating water by reverse osmosis, in particular for desalinating sea water.
2. Description of the Related Art
An apparatus of that kind is described for example in WO 02/41979 A1. In that apparatus the salt water is introduced under a first pressure into a pressure compensating device and from there passed under a second higher pressure into a membrane module. In the membrane module, it is separated into desalinated water and concentrated salt water. The discharged concentrated salt water which is approximately still at the second pressure is continuously introduced into the pressure compensating device again and is used therein for subjecting the salt water introduced into the pressure compensating device to approximately the second pressure and for introducing the salt water into the membrane module. More specifically the pressure compensating device described therein has two piston/cylinder devices which operate in opposite phase relationship and the pistons of which are fixedly connected together by a piston rod which is additionally driven.
In desalination installations of that kind which operate on the basis of the principle of reverse osmosis, separation into concentrated salt water and desalinated water is effected at a so-called ‘crossflow’ membrane disposed in the membrane module. In the case of such a membrane, the salt water introduced flows along the surface of the membrane while a part thereof passes as desalinated water (drinking water) in a direction perpendicularly thereto through the membrane. The mutually crossing flows of water are also referred to as ‘crossflow’. In that case the flow on the surface of the membrane also flushes away unwanted foreign bodies on the surface of the membrane and accordingly therefore provides for continuously cleaning of the membrane.
In the known configuration of the desalination apparatus having two piston/cylinder devices, a sufficiently high pressure is admittedly present at the moment of switching over the direction of movement of the pistons, to further press water through the membrane and thus produce desalinated water. It has been found however that the crossflow collapses at the time of switching over the direction of movement. As a result, at that moment the membrane is no longer sufficiently flushed so that the situation can involve salt molecules becoming concentrated on the surface of the membrane, and that can result in a rise in osmotic pressure and thus the operating pressure to the stage of a salt crust being formed on the surface of the membrane and operation being permanently interrupted.
U.S. Pat. No. 4 187 173 and EP 0 018 128 A1 disclose a method of and an apparatus for desalinating water on the basis of reverse osmosis, wherein a respective pressure compensating container is provided both in the feed water circuit and also—in U.S. Pat. No. 4 187 173—in the concentrate circuit. Those pressure compensating containers are in the configuration therein of pulsation dampers or differential pressure dampers, in which a piston is displaceable in a cylinder and subdivides the interior of the cylinder into two chambers. For discharge of feed water disposed in a chamber, it is provided therein that pressure is applied to the piston by means of concentrate introduced into the second chamber, and a spring disposed in that chamber.
FR 2 568 321 and EP 0 055 981 A1 disclose further apparatuses for and methods of reverse osmosis.